Keyed shafts are used in applications where shafts must be aligned for timing or other purposes. A common example of a keyed shaft which requires rotational positioning as part of service procedures is an automobile engine crankshaft.
General purpose wrenches are not suitable for turning crankshafts because they may scratch or deform the surface of the crankshaft or key. If an automobile engine crankshaft is scratched it may make it difficult to reattach the harmonic balancer or vibration dampener to the crankshaft.
Specialized ratchet arms are available which include a ratchet gear matched to a particular crankshaft and key combination. Such ratchet arms are reversible to rotate the shaft in either direction during assembly or repair. However, since the ratchet gear is matched to a particular size crankshaft it is of limited utility. Keyed shafts may vary widely in diameter. For instance, engine crankshafts for vehicles currently range in diameter from 3/4 inches to 1 1/2 inches. In automotive repair facilities which service a variety of engine types a set of dedicated ratchet arms must be maintained to cover the range of crankshaft sizes. The use of dedicated wrenches is both costly and time consuming.